When it comes to fat, what difference does color make? A lot, it seems, especially if you’re interested in losing it.
For years now, scientists have been studying so-called brown fat, a type of heat-generating fat that burns energy rather than storing it. It’s abundant in rodents, which can’t shiver to warm up. Human newborns also have it, to keep warm. By adulthood, however, humans have lost most of their brown fat stores — initially, researchers thought adults didn’t have any brown fat at all — but recent studies have found that adults retain small pockets of it in certain places, such as the upper back and side of the neck. The question is, Can brown fat actually burn calories and aid in weight loss?
A new study led by Andre Carpentier at the University Hospital of Sherbrooke in Canada suggests it can. Reporting in the Journal of Clinical Investigation, the scientists found that they were able to activate brown fat in adult men by exposing them to cold. Even more promising, they discovered that brown fat gobbles up white fat, the kind that accumulates in love handles and that most of us have too much of. At rest, the study found, the men burned more calories when they were cold — about 250 calories in three hours.
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Knowing that chilling the body triggers brown fat to mobilize could lead to an entirely new strategy for weight loss, the researchers suggested. Treatments could focus on activating brown fat without having to keep people in the cold.
Brown fat gets its name from its darker color (compared to white fat), which is due to the mitochondria packed inside its cells. Mitochondria are the body’s energy factories, turning glucose from the food we eat into the energy that cells need to perform their functions.
There have been earlier hints that brown fat might be a useful weapon against obesity. In 2009, researchers in Sweden and the Netherlands reported that participants who were left to chill in a cold room (set at 16 C or 61 F) for a couple of hours with one foot plunging in and out of a bracing ice bath showed higher rates of activity in their brown fat regions in a PET scan. Carpentier’s group confirms those results, and pushes them one step closer to becoming a useful weight loss strategy.
Still, many questions remain before those next steps can be taken. For one thing, why is it that obese people tend to have very little brown fat compared with lean people? It makes sense, given that brown fat may be a back up system for warming the body when all other sources of fuel, including white fat, have been exhausted. But could a lack of brown fat help explain, in part, why obese people are obese?
Also, a new study in mice finds that animals are able to turn their white fat brown by exercising. During exercise, the animals’ muscles released a newly discovered enzyme called irisin, which triggered the conversion. It’s not clear whether the same phenomenon is true in people, though humans do have the same protein. However, the brown fat that is easily observed in humans tends not to be the kind that is derived from white fat.
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It’s also probably worth remembering how researchers discovered that adults retained stores of brown fat in the first place: they were studying head and neck scans of patients with cancer and noticed that in addition to tumor sites, certain parts of the neck also showed higher rates of glucose consumption. Those turned out to be the brown fat stores, which were greater in cancer patients than in people without tumors.
So far, the research on brown fat leaves some critical questions about manipulating such fat — and the consequences of that intervention — unanswered. “We have proof that this tissue burns calories. … But what happens over the long term is unknown,” Carpentier told the New York Times.
Which means it’s probably too soon to join the Polar Bear Club or turn down the thermostat in your home.